The present invention relates to a memory device. More particularly, the present invention relates to a method for forming a pattern in a semiconductor device by employing a self-aligned double exposure technology.
In general, a semiconductor device such as dynamic random access memory (“DRAM”) includes numerous fine patterns. Such patterns are formed through a photolithography process. In order to form a pattern by a photolithography process, a photoresist (“PR”) film is coated over a target layer to be patterned. Next, an exposure process is performed to change solubility in a given potion of the PR film. Subsequently, a developing process is performed to form a PR pattern exposing the target layer. That is, the PR pattern is formed by removing the portion of which the solubility is changed, or by removing the portion of which the solubility is not changed. Later, the exposed target layer is etched using the PR pattern, and then the PR pattern is stripped to form a target layer pattern.
In the photolithography process, resolution and depth of focus (“DOF”) are two important issues. Resolution (R) can be expressed by Equation 1 below.
                              R          =                                    k              1                        ⁢                          λ              NA                                      ,                            (        1        )            wherein k1 is a constant determined by a kind and thicknesses of PR film, λ is a wavelength of light source, and NA stands for a numerical aperture of exposure equipment.
According to the above Equation 1, the shorter the wavelength (λ) of a light source is and the larger the NA of exposure equipment is, the finer is a pattern formed over a wafer. However, λ of a light source being used and the NA of exposure equipment have not kept abreast of recently rapid advances in integration of a semiconductor device. Therefore, resolution enhancement technology (“RET”) for improving resolution and DOF is being applied by incorporating diverse methods. For example, the RET technology includes phase shift mask (“PSM”), off-axis illumination (“OAI”), optical proximity correction (“OPC”) and the like. Besides, there is a technology called double exposure technique (“DET”) capable of forming a fine pattern over a wafer. Critical Dimension (CD) uniformity in the DET depends on overall overlay accuracy of a first exposure mask and a second exposure mask.
However, it is difficult to control the overlay of the first and second exposure masks to fall within the error range. Moreover, technical difficulties make it hard to achieve improvement of exposure equipment.